Can someone use garlic or onion powder? What are the different shitos ?
It is permitted to use garlic and onion powder, in line with the rulings of most authorities.
See Kovetz Beis Hillel (Vol. 1, p. 62), where many contemporary authorities are cited who debate this issue.
Sefer Amudei Ha-Bayis (Vol. 3, Part 4, no. 13) discusses the question at length, and writes that kashrus authorities are careful that the onion or garlic should be cooked before the morning of their peeling, to avoid the kashrus issue of leaving peeled garlic or onion overnight.
Shut Shevet Ha-Levi (6:311) is lenient on the matter, though Rabbi Ovadyah Yosef, who was initially lenient (Yabia Omer 2:7), later left the question unresolved (Halichos Olam 7 p. 217).